1. Field of the Invention
The present invention relates to a displacement detecting device adapted to detect displacement of a surface-to-be-measured with high accuracy with a non-contact sensor that uses light.
2. Description of the Related Art
Conventionally, displacement detecting devices are widely used as devices for measuring displacement and shape of a surface-to-be-measured.
Among such displacement detecting devices, there is a type of displacement detecting device adapted to irradiate light on a diffraction grating fixed to a surface-to-be-measured, and detect the phase change of a diffracted light generated accompanying the displacement of the object-to-be-measured.
For example, Japanese Patent Publication No. 4023923 (referred to as “Patent document 1” hereinafter) discloses an art in which a coherent light emitted from a light source is split into two coherent lights, and the two coherent lights are incident on a diffraction grating. Two diffracted lights generated by the two coherent lights incident on the diffraction grating are reflected by a reflection optical system so as to be incident again on the diffraction grating.
By such a method, the light once diffracted is reflected by the reflection optical system and diffracted again by the diffraction grating, and thereby it is possible to improve detection resolution.
Particularly, in the art disclosed in Patent document 1, the coherent light incident on the diffraction grating forms an image on the diffraction grating. Further, the diffracted light is reflected in such a manner that the diffracted light diffracted by the diffraction grating is collimated into a parallel light by an imaging section, so that the light is constantly perpendicularly incident on a reflector.
With such an arrangement, the deviation of the image position of the diffracted light on the grating can be prevented, and the deviation of the optical axis of the diffracted light can also be prevented. Thus, it is possible to perform position detection with high accuracy.